NIO网络编程中重复触发读（写）事件

一、前言

　　公司最近要基于Netty构建一个TCP通讯框架， 因Netty是基于NIO的，为了更好的学习和使用Netty，特意去翻了之前记录的NIO的资料，以及重新实现了一遍NIO的网络通讯，不试不知道，一试发现好多细节没注意，导致客户端和服务端通讯的时候出现了一些非常莫名其妙的问题，这边我记录下耗了我一晚上的问题~

二、正文

　　废话不多说，先上问题代码~

　　服务端：

package com.nio.server;

import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;
import java.util.Iterator;

public class NIOServer {
    private static Selector selector;
    private static ServerSocketChannel serverSocketChannel;
    private static ByteBuffer bf = ByteBuffer.allocate(1024);
    public static void main(String[] args) throws Exception{
        init();
        while(true){
            selector.select();
            Iterator<SelectionKey> it = selector.selectedKeys().iterator();
            while(it.hasNext()){
                SelectionKey key = it.next();
                if(key.isAcceptable()){
                    System.out.println("连接准备就绪");
                    ServerSocketChannel server = (ServerSocketChannel)key.channel();
                    System.out.println("等待客户端连接中........................");
                    SocketChannel channel = server.accept();
                    channel.configureBlocking(false);
                    channel.register(selector,SelectionKey.OP_READ);
                }
                else if(key.isReadable()){
                    System.out.println("读准备就绪，开始读.......................");
                    SocketChannel channel = (SocketChannel)key.channel();
                    System.out.println("客户端的数据如下：");

                    int readLen = 0;
                    bf.clear();
                    StringBuffer sb = new StringBuffer();
                    while((readLen=channel.read(bf))>0){
                        sb.append(new String(bf.array()));
                        bf.clear();
                    }
                    if(-1==readLen){
                        channel.close();
                    }
                    channel.write(ByteBuffer.wrap(("客户端，你传过来的数据是："+sb.toString()).getBytes()));
                }
                it.remove();
            }
        }
    }
    private static void init() throws Exception{
        selector = Selector.open();
        serverSocketChannel = ServerSocketChannel.open();
        serverSocketChannel.configureBlocking(false);
        serverSocketChannel.socket().bind(new InetSocketAddress(8080));
        serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);
    }
}

　　客户端：

package com.nio.client;

import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.SocketChannel;
import java.util.Iterator;

public class NIOClient {
    private static Selector selector;
    public static void main(String[]args) throws Exception{
        selector = Selector.open();
        SocketChannel sc = SocketChannel.open();
        sc.configureBlocking(false);
        sc.connect(new InetSocketAddress("127.0.0.1",8080));
        sc.register(selector,SelectionKey.OP_READ);

        ByteBuffer bf = ByteBuffer.allocate(1024);
        bf.put("Hi,server,i'm client".getBytes());

        if(sc.finishConnect()){
            bf.flip();
            while(bf.hasRemaining()){
                sc.write(bf);
            }

            while(true){
                selector.select();
                Iterator<SelectionKey> it = selector.selectedKeys().iterator();
                while(it.hasNext()){
                    SelectionKey key = it.next();

                    if(key.isReadable()){
                        bf.clear();
                        SocketChannel othersc  = (SocketChannel)key.channel();
                        othersc.read(bf);
                        System.out.println("服务端返回的数据："+new String(bf.array()));
                    }
                }
                selector.selectedKeys().clear();
            }
        }
    }
}

　　服务端运行结果：

　　客户端运行结果：

　　这边我们可以看到，客户端输出了两次，笔者调试的时候发现，服务端只往客户端写过一次数据，但是客户端却打印了两次数据，而且两次的数据不一样，挺诡异的！然后我就各种查，折磨了我一夜，今早一来，又想着怎么解决问题，不经意间发现了一篇文章：java nio使用的是水平触发还是边缘触发？，文章中指出Nio的Selector.select()是“水平触发”（也叫“条件触发”），只要条件一直满足，那么就会一直触发，至此我如醍醐灌顶：是不是我通道里面的数据第一次没有读取干净？导致客户端触发了多次读取？后来验证之后，发现确实是这个问题，读者可以看我服务器端的代码，我返回的是数据字节数是："服务端返回的数据：".length()+bf.array().length=26+1024，而客户端只是将这个数据读入1024大小的ByteBuffer中，还有26字节没有读取干净，所以就触发了第二次的读事件！！！

　　既然问题找到了，现在就是要解决如何将通道内的数据读取干净了，修改之后的代码如下， 特别注意红色部分：

　　服务端：

　　

package com.nio.server;

import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;
import java.util.Iterator;

public class NIOServer {
    private static Selector selector;
    private static ServerSocketChannel serverSocketChannel;
    private static ByteBuffer bf = ByteBuffer.allocate(1024);
    public static void main(String[] args) throws Exception{
        init();
        while(true){
            selector.select();
            Iterator<SelectionKey> it = selector.selectedKeys().iterator();
            while(it.hasNext()){
                SelectionKey key = it.next();
                if(key.isAcceptable()){
                    System.out.println("连接准备就绪");
                    ServerSocketChannel server = (ServerSocketChannel)key.channel();
                    System.out.println("等待客户端连接中........................");
                    SocketChannel channel = server.accept();
                    channel.configureBlocking(false);
                    channel.register(selector,SelectionKey.OP_READ);
                }
                else if(key.isReadable()){
                    System.out.println("读准备就绪，开始读.......................");
                    SocketChannel channel = (SocketChannel)key.channel();
                    System.out.println("客户端的数据如下：");

                    int readLen = 0;
                    bf.clear();
                    StringBuffer sb = new StringBuffer();
                    while((readLen=channel.read(bf))>0){
                        bf.flip();
                        byte [] temp = new byte[readLen];
                        bf.get(temp,0,readLen);
                        sb.append(new String(temp));
                        bf.clear();
                    }
                    if(-1==readLen){
                        channel.close();
                    }
　　　　　　　　　　　　System.out.println(sb.toString());
                    channel.write(ByteBuffer.wrap(("客户端，你传过来的数据是："+sb.toString()).getBytes()));
                }
                it.remove();
            }
        }
    }
    private static void init() throws Exception{
        selector = Selector.open();
        serverSocketChannel = ServerSocketChannel.open();
        serverSocketChannel.configureBlocking(false);
        serverSocketChannel.socket().bind(new InetSocketAddress(8080));
        serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);
    }
}

　　客户端：

　　

package com.nio.client;

import java.io.ByteArrayOutputStream;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.SocketChannel;
import java.util.Iterator;

public class NIOClient {
    private static Selector selector;
    public static void main(String[]args) throws Exception{
        selector = Selector.open();
        SocketChannel sc = SocketChannel.open();
        sc.configureBlocking(false);
        sc.connect(new InetSocketAddress("127.0.0.1",8080));
        sc.register(selector,SelectionKey.OP_READ);

        ByteBuffer bf = ByteBuffer.allocate(1024);
        bf.put("Hi,server,i'm client".getBytes());

        if(sc.finishConnect()){
            bf.flip();
            while(bf.hasRemaining()){
                sc.write(bf);
            }

            while(true){
                selector.select();
                Iterator<SelectionKey> it = selector.selectedKeys().iterator();
                while(it.hasNext()){
                    SelectionKey key = it.next();

                    if(key.isReadable()){
                        ByteArrayOutputStream bos = new ByteArrayOutputStream();
                        bf.clear();
                        SocketChannel othersc  = (SocketChannel)key.channel();
                        while(othersc.read(bf)>0){
                            bf.flip();
                            while(bf.hasRemaining()){
                                bos.write(bf.get());
                            }
                            bf.clear();
                        };
                        System.out.println("服务端返回的数据："+bos.toString());
                    }
                }
                selector.selectedKeys().clear();
            }
        }
    }
}

　　客户端的输出：

　　 

三、参考链接

https://www.zhihu.com/question/22524908

四、联系本人

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